Additives for controlling the physical and structural characteristics of cast iron

ABSTRACT

COMPOSITIONS OR MIXTURES OF SILICON CARBIDE, COLUMBITE (A MINERAL OF THE NIOBIUM-IRON TANTALATE GROUP) AS WELL AS OTHER SUBSTANCES SUCH AS CHROMITE, THIXOTROPIC AND SURFACE-ACTIVE INGREDIENTS HAVE BEEN ADDED TO EFFECT THE HARDNESS AND TEXTURAL OR STRUCTURA HOMOGENEITY OF CAST IRON AND FERROUS ALLOYS. SILICON CARBIDE AND BADDELEYITE (A MINERAL OF ZIRCONIUM DIOXIDE, FOUND AT CALDAS, MINAS GERAIS, BRAZIL) PELLETED WITH OTHER MATERIALS HAS ALSO BEEN FOUND USEFUL AS AN ADDITIVE TO CAST IRON. ANOTHER ADDITIVE IS FORMED OF SILICON CARBIDE AND A NODULIZING (SPHERULIZING) AGENT, PARTICULARLY WITH METALLIC MAGNESIUM PELLETED WITH ADDITIONAL SUBSTANCES, SUCH AS THIXOTROPIC AND SURFACE ACTIVE AGENTS.

United States Patent 3,744,998 ADDITIVES FOR CONTROLLING THE PHiiSiQAL AND STRUCTURAL QHARAQTERTSTTC 01F CAST IRON Domingos Loriechio, Sao Paulo, Brazil, assignor to The Carborundum Company, Niagara, Falis, I LY. No Drawing. Filed Feb. 16, 1971, Ser. No. 115,695 Int. Ci. C22c 37/02, 37/04 US. Cl. 752i30 R 9 Claims ABSETRACT OF THE DISCLOSURE Compositions or mixtures of silicon carbide, columbite (a mineral of the niobium-iron tantalate group) as well as other substances such as chromite, thixotropic and surface-active ingredients have been added to effect the hardness and textural or structural homogeneity of cast iron and ferrous alloys. Silicon carbide and baddeleyite (a mineral of zirconium dioxide, found at Caldas, Minas Gerais, Brazil) pelleted with other materials has also been found useful as an additive to cast iron. Another additive is formed of silicon carbide and a nodulizing (spherulizing) agent, particularly with metallic magnesium pelleted with additional substances, such as thixotropic and surface active agents.

This invention relates to additives for controlling the physical and structural characteristics of cast irons and more particularly to pelletized mixtures containing silicon carbide.

BACKGROUND OF THE INVENTION In a prior application filed in Brazil on Dec. 17, 1968 with Ser. No. 204,905 and filed Feb. 16, 1971 in the United States (Ser. No. 115,694) in the name of the present applicant entitled: A Process for the Production of Cast Irons and Additives Therefor, now US. Pat. 3,682,625 the applicant discloses certain improvements relating to the production of cast irons, and certain additives for homogenizing the microstructure and controlling the hardness of cast iron products. The applicant has now found that by applying other additives to cast iron one can obtain further technical advantages.

As used in the present application cast iron includes any alloy based on carbon iron which contains more than one and seven tenths (1.7%) percent by weight total carbon and up to about four (4.0%) percent by weight total carbon. Such alloys may normally contain other elements such as from five hundreths of one (0.05%) percent by weight to two tenths (0.2%) percent by weight sulfur, from five tenths (0.5%) percent by weight to three (3.0%) percent by weight silicon, from five tenths (0.5%) percent by Weight to one (1.0%) percent by weight manganese and from one tenth (0.1%) percent by Weight to one (1.0%) percent by weight phosphorous.

A variety of such alloys are useful for varying purposes however not all have the most desirable features for specific applications. Accordingly, it is desirable to improve the structural and textural features of the cast iron after commencing the production thereof. In this manner the properties and even the application of the finished products made with a particular cast iron can be modified.

Means for modifying the hardness and structural homogeneity by the inclusion of additives into the charge itself of the starting ferrous alloy in furnaces are presently known. For example the properties of cast iron may be modified by the addition of scrap iron to the cupola or by the addition of corrective additives and/or by catalytical action on the microstructure of the cast iron, particularly in the pan (ladle or forecrucible). The processing in the furnace or cupola can thus be made without extreme care by monitoring the composition.

33%,098 Patented July 10, 1973 The technological importance of utilizing such additives is apparent. The additives may be varied for obtaining particular modifications, for example to inhibit the formation of hard points due to segregation of cementite Fe c.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The additives according to the present invention may also be applied to steels, particularly mild steels having a carbon content in the range of from five tenths (0.5%) percent by weight to about one and seven tenths (1.7%) percent by Weight. More particularly the present invention relates to compositions or mixtures in the pelleted form or pellets containing silicon carbide, columbite (a mineral of the niobium-iron tantalate group) and other substances such as chromite and thixotropic and surfaceactive ingredients.

An additional additive according to the present invention comprises silicon carbide, baddeleyite (a mineral comprising zirconium dioxide, found at Caldas, Minas Gerais, Brazil), pelleted with additional substances such as indicated above.

Another additive according to the present invention is formed with silicon carbide and a nodulizing (spherulizing) agent, particularly with metallic magnesium in the pelleted form. Other additives such as thixotropic and surface-active substances are preferably included.

DETAILED DESCRIPTION Additives including columbite in the pelleted form may be prepared in accordance with the following procedure. Silicon carbide having a high activity and a particle size corresponding to a surface area in the range of fifteen (15) to thirty five (35) square meters per gram which is equivalent to an mesh screen (Tyler) up to a 'rnicronized powder is utilized. Columbite and optionally chromite which is useful for correcting hardness are applied in a finely pulverized form. The finely pulverized form is preferably less than forty (40) mesh Tyler screen in colloidal condition, dispersed in an aqueous suspension. The aqueous suspension includes a conventional thixotropic and/or surface-active ingredient.

The surface-active agent should not include incompatible inorganic elements such as sulfurated or phosphorated compounds. The agents modify the surface attraction and repulsion forces of the silicon carbide, chromite and/or columbite particles so that they fit into or mutually interpenetrate into the respective pores. If the aqueous suspension of columbite and colloidal chromite does not already contain a thixotropic agent, the same is added to the sus pension preferably after the incorporation of the silicon carbide therein.

These ingredients are blended intensely and the mass is left standing to cure. The fairly thick mass is then extruded into strings which are cut into proper size or pelleted on a rotary dish. In the alternative they may be granulated by passing the mass through coarse screens. During the extrusion step, a slow and uniform extrusion is employed under a pressure of about 15 kg./cm. to improve the consistency of the final dried pellets.

The additives have the proportion of columbite or columbite+chromite to silicon carbide in the range of 5:1 to 1:2, while the proportion of columbite to chromite may vary from 10:1 to 1:10. All of the aforementioned proportions are expressed by weight.

In the composition, the thixotropic agent is preferably present in a proportion of about 1 to 5% by weight and the surface-active agent amount to 0.1 to 1%, and prefer ably about 0.5% by Weight.

At times it may be desirable to obtain a greater temporary consistency to the aqueous mass. Binding agents such as dextrine, cellulose ethers and esters and vegetable gums may be used in the range of 0.5 to 1.5% by weight for this purpose. These agents are not essential to the practice of the present invention.

The dry pelleted product is added to irons and steels at the jet, i.e., at the casting from the furnace to the pan or ladle. The efiect is to control the size of the structural grain, refining, homogenizing and densifying the microstructure of the alloy.

One experiment carried out by the applicant has shown a noticeable increase in tensile strength. The efiect has been particularly noticeable in alloys for manufacturing pipes which are capable of withstanding high hydraulic pressures or heavy tensile stresses by tensioning or bending. Experiments also showed that additions of the novel additive or composition to chrome steels which are suitable for springs, at the pan stage improve their strengths in yield point, in permanent set and in rupture.

The additives which include silicon carbide and baddeleyite as essential elements or components have the specific purpose of inoculating silicon and zirconium into cast iron. This is particularly important in the presence of iron-manganese in cast irons which must present high mechanical strengths and high corrosion resistance.

One experiment carried out by the applicant has shown that the zirconium dioxide, ZrO of the baddeleyite is reduced in situ by silicon carbide. It has also been shown that inoculating elemental zirconium Zr into the alloy produces the desired chemical and physical features.

The silicon carbide and baddeleyite granules were formed in the same manner as set forth above for forming the additives with columbite.

The nodulizing additive according to the present invention is also based on silicon carbide. The silicon carbide serves as an agent or carrier for introducing a nodular carbon or nodulizing agent, such as, metallic magnesium, into cast irons.

Magnesium has in accordance with conventional practice been added to an alloy as a nodulizing agent, i.e., as a nodular graphitic carbon forming nucleus or a carbon nodulizing nucleus. This practice has been used in the production of nodular cast irons of greater malleability. However, in view of the low density and high chemical reactivity of magnesium, a substantial amount of the magnesium is lost by oxidation or burning before obtaining its nodulizing action.

The practice of the present invention overcomes the aforementioned disadvantage by the provision of pellets or granules based on silicon carbide. The silicon carbide which may be of two types: alpha and beta, is mixed with metallic magnesium. The relative proportions of the siliclon carbide to the magnesium is in the range of :1 to :1.

The pellets or granules are prepared as set forth above, except that a silicon carbide of mixed alpha+beta crystallization is employed. The silicon carbide which should have a particle size between 12 mesh Tyler and 80 mesh Tyler screens is mixed in an aqueous suspension with thixotropic and surface-active ingredients and auxiliaries. Powdered magnesium is then added and the resulting mixture pelleted or granulated as indicated above.

The granules or pellets based on silicon carbide and magnesium are eifective to protect the magnesium against immediate or premature action and facilitate the inoculating action of the magnesium on the cast iron. As a practical result, a lower percentage of metallic magnesium can be used for obtaining a nodulization effect on cast iron. This fact arises from the the protection afforded to the metal magnesium in the granules by their envelopment or covering with silicon carbide crystals. The silicon carbide protects them from premature oxidation. Furthermore, silicon carbide deoxidizes the metal in loco where magnesium is found, appeases the melt ambient, affords the rapid action of the inoculating metal with full profit and ensures the nodulization of cast iron at a lower less cost.

What is claimed is:

1. An additive for controlling the structural and physical properties of cast iron comprising a pelletized mixture of silicon carbide and a complementary agent, said complementary agent selected from the group consisting of columbite, baddeleyite, and columbite with chromite, wherein the ratio of silicon carbide to the complementary agent is from ten (10) parts silicon carbide to one (1) part complementary agent to one (1) part silicon carbide to one (1) part complementary agent on a by weight basis.

2. An additive according to claim 1 which includes a thixotropic agent in an amount of about one to five percent by weight and a surface-active agent in an amount of about one tenth of one percent to one percent by weight.

3. An additive according to claim 2 in which the complementary agent is a finely divided and intimately mixed blend of columbite and chromite wherein the ratio of columbite to chromite is from ten parts columbite to one part chromite to ten parts chromite to one part columbite.

4. An additive according to claim 1 in which the complementary agent is columbite.

5. An additive according to claim 1 in which the complementary agent is baddeleyite.

6. An additive according to claim 1 in which the complementary agent is columbite with chromite.

7. A method for controlling the structural and physical properties of cast iron with an additive mixture, the method comprising the steps of (a) forming a mixture of pulverized silicon carbide and complementary agent, said agent being selected from the group consisting of columbite, baddeleyite, columbite with chromite, and magnesium;

(b) dispersing the mixture with a thixotropic agent and a surface-active agent in an aqueous suspension;

(0) blending the ingredients to form an extrudable mass;

(d) extruding the mass and drying the extruded material; and

(e) adding the dried extruded material to molten iron, whereby the properties of the iron, when cast, are improved.

8. A method according to claim 7 in which the ratio by weight of silicon carbide to the complementary agent ranges from ten parts of silicon carbide to one part of complementary agent, to one part of silicon carbide to one part of complementary agent.

9. A method according to claim 7 in which the complementary agent is magnesium.

References Cited UNITED STATES PATENTS 933,357 9/1909 Baraduc-Muller -53 X 2,465,672 3/ 1949 Blaha 106-44 2,569,146 9/1951 Bolkcom 75-130 A X 2,889,218 6/1959 Hiskey et a1. 75-27 L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner 

